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ACI 214R Document Information:
Title
Evaluation of Strength Test Results of Concrete
American Concrete Institute
Publication Date:
Jun 27, 2002
Scope:
INTRODUCTION
This document provides an introduction to the evaluation of concrete
strength tests. The procedures described are applicable to the
compressive-strength test results required by ACI 301, ACI 318, and
other similar specifications and codes. The statistical concepts
described are applicable for analysis of other common concrete test
results including flexural strength, slump, air content, and density.
Most construction projects in the United States and Canada require
routine sampling and fabrication of standard molded cylinders. These
cylinders are usually cast from samples of concrete taken from the
discharge of a truck or a batch of concrete and molded, cured, and
tested under standardized procedures. The results represent the
potential strength of the concrete rather than the actual strength of
the concrete in the structure.
Inevitably, strength test results vary. Variations in measured strength
may originate from any of the following sources:
Batch-to-batch variations of the proportions and characteristics of
the constituent materials in the concrete, the production, delivery, and
handling process, and climatic conditions; and
Variations in the sampling, specimen preparation, curing, and
testing procedures (within-test)
Conclusions regarding the strength of concrete can only be derived from
a series of tests. The characteristics of concrete strength can be
estimated with reasonable accuracy only when an adequate number of tests
are conducted, strictly in accordance with standard practices and test
methods.
Statistical procedures provide tools of considerable value when
evaluating the results of strength tests. Information derived from such
procedures is also valuable in refining design criteria and
specifications. This report discusses variations that occur in the
strength of concrete and presents statistical procedures that are
useful in the interpretation of these variations with respect to
specified testing and acceptance criteria.
For the statistical procedures described in this report to be valid, the
data should be derived from samples obtained by means of a random
sampling plan designed to reduce the possibility that selection will be
exercised by the sampler. Random sampling means that each possible
sample has an equal chance of being selected. To ensure this condition,
the selection should be made by some objective mechanism such as a table
of random numbers. If sample batches are selected on the basis of
judgement by the sampler, biases are likely to be introduced that will
invalidate the analysis using the procedures presented here. Natrella
(1963) and ASTM D 3665 provide a discussion of random sampling and a
useful short table of random numbers.
This report begins with a discussion of the sources of variability in
concrete as produced, mixed, and transported, and the additional
variability of samples obtained from the concrete as delivered and
tested. The report then describes the statistical tools used to
evaluate the variability of concrete and determine compliance with a
given specification, including both random variation and variation due
to assignable causes. Statistically based specifications are also
reviewed.
Keywords:
- coefficient of variation
- quality control
- standard deviation
- strength
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